This invention relates to a method of treating an ullage tape having a polymeric surface to prevent the build-up of electrostatic charge therein and to an ullage tape having an anti-static coating therein.
More particularly, the present invention is concerned with a method of coating an ullage tape to provide carbon black or lamp black thereon and to an ullage tape having carbon black or lamp black coated thereon to prevent static charge build-up on the tape.
In applicant's earlier U.S. Pat. No. 4,123,753, there is disclosed a metallic tape which provides for the leakage of charges accumulated on the outer jacket to be leaked therethrough for dissipation to the ground.
Although the present invention is generally applicable to the application of a carbon black coating to various polymeric substrates, it is particularly applicable to the coating of "Tefzel" fluoropolymer film tapes used to form ullage measuring devices.
The type of ullage measuring device to which the present invention is intended to be applied, is basically the weighted tape type of device wherein a weight pulls a tape down into a large liquid cargo tank until it reaches the surface of the liquid. The amount of liquid necessary to fill the rest of the tank can be estimated from the depth of the level of liquid in the tank. While the coating is particularly intended for the tape used in applicant's "Ullage Measuring Device" disclosed in the aforenoted U.S. Pat. No. 4,123,753, it is also particularly useful wherever electrostatic charge build-up on a "Fluorocarbon" material is to be prevented.
Static charge build up is a particular problem in large tanks which are in tanker ships used for carrying flammable liquids. The danger of explosion is high as the liquid inside the tank can be ignited from a spark caused by static electric charges. It is not uncommon for large tankers to have generated high static charges in the vapor over the liquid carried in the tanks during loading and cargo handling.
As is mentioned in U.S. Pat. No. 4,123,753, at column 3, lines 13-29, the device disclosed therein contemplates the use of a tape assembly or construction which has an outer protective covering of insulating material which is knurled or dimpled so as to provide for an irregular surface cover for the metallic tape. A tape construction with an irregular covering surface is especially useful in those situations where there is a large static charge build-up. As further noted in column 3 of U.S. Pat. No. 4,123,753, this construction helps to reduce static charge build up because the charge leaks through and is dissipated by the metal tape into the dimpled insulating surface protection cover.
Liquid antistatics are clear liquids, making it difficult to determine if the tape is safely coated. My coating produces a light haze color to show the tape is coated. It has also been suggested that various antistatic coatings be sprayed onto the surface of the tape to reduce or neutralize static charges thereon. However, in practice, these have been found to be impractical. Although liquid antistatic agents are reasonably efficient, they are removed when the tape is cleaned between measurements. Thus, although such antistatic agents may work for a single measurement, the use of the tape for measuring a number of tanks one after another becomes messy and awkward, particularly as is usual when a ship with 20 tanks is to be measured. After each measurement, the tape must be cleaned and then recoated before it can be used for the next measurement.
The use of carbon black to give polymeric materials sufficient conducting properties to disperse static electric charges is known. However, the application of a layer by a method analogous to the method of the present Application, and the use of a carbon layer in conjunction with ullage tapes or the like, appears unknown.
U.S. Pat. No. 2,997,451 discloses a method whereby various amounts of carbon black, made by many different methods, are mixed into thermoplastic resins to give the resins sufficient conductivity to reduce "static" when phonograph records are played. U.S. Pat. No. 3,249,559 teaches the application of a coating of graphite to polytetrafluoroethylene (PTFE) by using a solvent carrier.
U.S. Pat. No. 3,753,765 teaches the application of a dispersed carbon black pigment layer to the black of a polymeric photographic film support. But, according to the disclosure, the use of a binder is specifically required.
The disclosure of U.S. Pat. No. 3,865,626, is similar to the disclosure of U.S. Pat. No. 3,249,559 in that the polymer is treated with a solvent or other swelling agent to cause the carbon particles to adhere to the surface. It is further disclosed that the surface may be heat treated to improve the anchoring of the layer to the surface of the polyester material.
In summary, although it is known to use carbon black to impart conductivity to various polymeric items, the use of such a method for ullage tapes or the like is not shown or suggested. In addition, the method of applying carbon to elongated polymeric strips and in particular to elongated polytetrafluoroethylene or "Tefzel" (registered trademark of E.I. Dupont DeNemours & Company), coated tapes using a fuel rich acetylene torch to form and anchor the carbon directly onto the strip in one step, as it passes by the torch, is not shown or suggested in the prior art. This process produces a stable conductive coating.
It is therefore an object of the present invention to provide an efficient static electric charge dispersing coating which is permanent and will not rub off during normal cleaning of the tape of an ullage measuring device.
It is a further object to provide a simple and efficient method of applying such a static electric charge removal coating to an ullage measuring device.
It is a related object to provide a method for applying a carbon black coating to polymeric material without the need for special solvents or adhesive bases. It is a still further object to apply such a coating in particular polytetrafluoroethylene.
Yet another object of the invention is to provide an ullage tape with a carbon black coating to prevent the buildup thereon of anti-static charges.
Other objects, advantages and the nature of the invention will become readily apparent from the detailed description of the invention in connection with the accompanying drawings.